1. Field of Invention
This invention relates to fluid flow sensors and has particular, but not exclusive, reference to flow sensors for use in hydraulic controls.
2. Description of the Prior Art
In British patent specification No. 1335041 there is disclosed an hydraulic actuator control embodying a flow sensor which is used to provide hydraulic feedback. In some instances the flow sensor comprises a spool or bobbin which is movable within a profiled throat to effect the required flow sensing and in one embodiment, the bobbin is shaped at each end and is acted upon at each end by a spring, with the throat being a regular cylinder, i.e. not profiled. The use of a profiled throat has been found to be relatively expensive in that the machining operation required is not entirely straightforward. Furthermore, if the characteristics of the flow sensor have to be changed, it is necessary to provide a throat with a different profile and hence change the whole body of the sensor. Nevertheless, this has been the preferred procedure rather than use the shaped bobbin because the latter is relatively long and flow sensors often need to be mounted where space is at a premium. Also, the use of hydraulic feedback from the flow sensor in this prior arrangement does not permit any adjustment of the gain of the system which can be effected using electrical feedback.